The broad aim of this grant is to understand cochlear mechanics by performing direct measurements of vibration of the basilar membrane, tectorial membrane or Reissner's membrane in cat, guinea pig and gerbil. Measurements will be made with a displacement-sensitive interferometer that allows both DC and AC measurements simultaneously, with greater speed, sensitivity, dynamic range, and bandwidth than older techniques. Accurate middle ear transfer functions will be provided as an essential element for cochlear modelers. Initial studies in the hook region of the cochlea revealed mechanical nonlinearities in the vibration of the basilar membrane. We will extend our initial observations of vibration in the hook region to include detailed transfer functions in both the radial and longitudinal axis of the basilar membrane. Both two-tone distortion (2TD) and two-tone suppression (2TS) will be studied. In the middle turn of the cochlea we will make similar measurements. In the apical turn, the AC and DC transfer functions along with 2TD and 2TD will be studied for Reissner's membrane, tectorial membrane and Hensen's cells. The effect of noise and of drugs thought to act upon outer hair cells on the mechanics of the cochlea will be studied. A goal of these experiments is to understand the role of the tectorial membrane in cochlear mechanics. To test conclusions from mechanical measurements independently, recordings will be made from auditory nerve fibers. Recordings will examine 2TS in high frequency fibers in cat and for a variety of nonlinear behavior in low frequency fibers in guinea pig. How the non-linear mechanical system of the cochlea contributes to the processing of complex, natural sounds will also be examined. Mechanical motion will be measured when the cochlea is stimulated with amplitude modulated signals, harmonic complexes, and speech fragments. Low- frequency biasing signals will clarify the role of the compressive nonlinearity and the time contants associated with suppression. The effect of the efferent system on mechanics will be addressed by activating the crossed olivocochlear system with sound contralaterally: With this paradigm frequency, intensity, and temporal effects are to be explored.